In milling certain types of materials it is often necessary or desirable to have a positive control of the atmosphere within the mill at all times. For example, readily oxidizable materials such as aluminum, titanium, magnesium, lithium and fine powders of many compositions are combustible or even explosive under certain conditions or they may be contaminated by the presence of air. In milling such materials the control of the atmosphere must extend to charging and discharging of the mill without opening the mill to air.
The problems encountered in milling powders are particularly troublesome in the mechanical alloying of readily oxidizable metals such as aluminum, magnesium and lithium. Mechanical alloying has been described in detail in the literature and in patents. U.S. Pat. Nos. 3,740,210, 3,816,080 and 3,837,930, for example, involve the mechanical alloying of aluminum alloys and other composite materials containing aluminum. In the practice of mechanical alloying the components of the product are charged in powder form into a high energy milling device such as a ball mill where, in an environment free of or reduced in amount of free or combined oxygen, the powders that are dry or substantially dry are ground down to a very fine size initially, prior to particle agglomeration in the latter stages of the process. This initial grinding increases the total surface area of the metallic powders significantly. Since any freshly exposed surface is not oxidized, it is very hungry for oxygen to the extent that the powders in this condition will burn and/or might explode spontaneously if exposed to air. Thus, any port in the mill, for example, for charge or discharge of powders, is a source of potential danger from the standpoint of the quality of the product produced and the possibility of a fire and/or an explosion.
The present invention is particularly useful for charging mills in a manner which will protect the charge material and the environment in the mill during loading of the mill. The system is also designed to minimize the problem of exposure to the environment during operation of the mill. For that reason the invention is described herein with reference to metal powders which are readily oxidized and are prepared as dispersion strengthened materials or alloys by powder metallury routes. Of necessity the milling of such materials must be carried out in a controlled atmosphere. It will be understood, however, that the present invention is not restricted to the processing of any materials or any particular milling or grinding apparatus. The loading design lends itself to a wider application. In ball mills, for example, it has been conventional to provide loading devices which are normal to the mill; that is when the mill shell is positioned for loading the loading device is in a vertical position with the charge port at the top of the shell. In the improved design of this invention the loading device is in a vertical position when the mill shell is rotated so that the port for entry of the charge material is on the side of the mill, with unencumbered entry for the feed material into the shell.
The charging system of the present invention can be incorporated into existing mills. Improved means for discharging a mill which can be used are disclosed in co-pending U.S. applications Ser. Nos. 712,703 and 712,704, now Pat. No. 4,603,814, filed simultaneously herewith.